After an aircraft lands on a runway at an airport or airfield, the aircraft must travel to an arrival destination, such as a gate or other parking location. An aircraft's travel path from touchdown to the point where passengers and/or cargo can be unloaded typically requires the aircraft to change its direction of travel as it moves along the ground. Virtually all commercial sized aircraft use the nose landing gear wheels to effect changes in ground travel or taxi direction. Most aircraft have a pair of nose landing gear wheels at the forward end of the aircraft connected with a steering system that enables a pilot in the aircraft cockpit to control the movement of the nose wheels to the right or to the left as needed to move the aircraft in a required right or left direction. Typically, hydraulic power is used to turn the nose wheels in response to pilot input from rudder pedals, a tiller wheel, or both. The steering systems of different kinds of aircraft may use specific variations of these components.
Many aircraft accomplish steering by swiveling a lower portion of the shock strut of the nose landing gear wheels. A hydraulic steering unit is usually mounted on a fixed portion of the shock strut and is linked to a swiveling portion of the landing gear structure to which the nose wheel or nose wheels are attached. Typically, a hydraulic steering unit includes valves and other components that enable the steering system to act as a shimmy damper when it is not used for steering. The nose wheel steering system is linked to rudder pedals in the cockpit, which are activated to turn the aircraft in a desired direction. Activation of the rudder pedals can turn the nose wheels through only a relatively small steering angle, however. If it is necessary to turn the aircraft through a greater steering angle, differential braking is usually used. In this event, the steering unit will be disengaged so that the nose wheels swivel freely.
In a Boeing 737, a hydraulic system is used in combination with both rudder pedals and a tiller wheel to turn the nose wheels to either side over a range of from zero degrees to about 78°. An interconnect mechanism enables control of steering by both rudder pedals and a tiller wheel. The tiller wheel provides the maximum steering and direction change of the nose wheels up to about 78°, while the rudder pedals provide steering when small directional changes are required. Full deflection of the rudder pedals produces about 7° of nose wheel steering. The rudder pedals are engaged to steer the nose wheels only when the aircraft is traveling on the ground between landing and takeoff. Squat switches and the like are included on the nose landing gear to ensure that the rudder pedals are operable to steer the nose wheels only when the aircraft is on the ground and are disengaged when the aircraft is airborne.
Other aircraft steering systems, such as that employed by the Airbus 320 aircraft, use electrical controls in combination with the aircraft's rudder pedals and tiller wheel to control steering during ground travel. The position of the nose landing gear wheels is measured by a transducer, which may be a linear or rotary variable differential transducer. Information relating to nose wheel position is sent to a brake and steering control unit and is compared to tiller or rudder input to produce a nose wheel steering angle. The nose wheels can be turned up to 75° manually by the tiller wheel. A hydraulic valve in the steering hydraulic system is commanded to send more or less pressure to a hydraulic actuating cylinder to move the nose wheels as commanded. This occurs when appropriate switches are on, a towing control lever is in a normal position, and at least one engine is running when the aircraft is on the ground.
Aircraft steering systems must be deactivated when aircraft are pushed back from a departure gate or location, towed, or otherwise moved on the ground by attached tugs or tow vehicles. This requires cooperative action by both ground crew and the aircraft's cockpit crew to ensure that landing gear steering is disabled and signals from rudder pedals and other steering system components are prevented from reaching steering system controls.
As indicated above, aircraft steering systems can be electrically or mechanically controlled and typically include a hydraulic system that may be electrically actuated to control steering angle. Rudder pedal steering, by itself, may not allow the nose wheel steering needed to maneuver on all airport taxiways or within ramp areas. In some aircraft, turns of a greater steering angle than is possible with rudder pedals require the disengagement of the steering unit so that the nose wheels can swivel freely, which can be done automatically. Achieving a greater angle of turn than the 7° possible with rudder pedals could require the use of differential braking and/or thrust. The use of differential braking forces to change travel direction while an aircraft's engines are operating to move the aircraft on the ground after landing is disclosed in U.S. Pat. No. 6,671,588 to Otake et al.
Available aircraft steering systems are used on aircraft that rely on the operation of one or more of the aircraft's main engines to move the aircraft during ground travel. Moving an aircraft autonomously on the ground without reliance on the aircraft's main engines or tow vehicles has been proposed. U.S. Pat. No. 7,469,858 to Edelson; U.S. Pat. No. 7,891,609 to Cox; U.S. Pat. No. 7,975,960 to Cox; U.S. Pat. No. 8,109,463 to Cox et al; and British Patent No. 2457144, owned in common with the present invention, describe aircraft drive systems that use electric drive motors to power aircraft wheels and move an aircraft on the ground without reliance on aircraft main engines or external vehicles. These drive systems effectively move aircraft autonomously during ground travel between landing and takeoff and at other times. It is not suggested that these drive systems could interact with an aircraft's nose wheel steering system to provide more effective or improved nose wheel steering.
The self-contained taxi system described by Kelly et al in U.S. Pat. No. 3,807,664 includes a mechanism connected to an aircraft's main wheels that controls wheel drive speed and torque to drive aircraft wheels at taxi speed and an aircraft's electro-hydraulic steering system to control nose wheel steering during taxi. Control of aircraft movement and nose wheel steering is accomplished primarily by regulating hydraulic fluid flow, and it is not suggested how the wheel drive speed/torque mechanism described could affect nose wheel steering, at low or other travel speeds.
A need exists for a system capable of controlling nose wheel steering, especially at low speeds, in an aircraft equipped with a wheel drive system that enables the aircraft to be driven autonomously on the ground without reliance on the aircraft's main engines or external tow vehicles.